Middle ultraviolet imager observations of the distribution of polar mesospheric clouds

Abstract

Abstract Observations of the spatial structure and distribution of Polar mesospheric clouds (PMCs) in the northern and southern polar regions were made during the arctic summers of 1998–1999 and austral summer of 1997–1998 by a middle ultraviolet imager (235–263 nm). The imager look-point made multiple transpolar passes over the Arctic and Antarctic and obtained thousands of images of PMCs at latitudes poleward of 55°. The unprecedented accuracy and stability of the satellite platform (∼10 μrad) allowed investigation of both the large-scale (100 km) and small scale (∼1 km) spatial structures of PMCs. The UV imager readily observed PMCs distinct from the atmospheric backgrounds. The clouds existed in discrete, filamentary structures throughout the polar region above latitudes of ∼55°. Vertically, PMCs generally lie between altitudes of 82.0 and 83.0 km, but within this range, cloud altitudes are essentially random on transpolar scales of ∼1000 km. In some instances, the clouds cluster at common altitudes for distances of ∼100 km. The PMC altitudes do not appear correlated with latitude or local time. A diffuse signal above the clouds, possibly related to Polar Mesospheric Summer Echoes, was observed in some cases. Horizontally, PMCs exhibit structure on scales of ∼50 km to ∼1000 km that is probably related to gravity wave activity in the polar region. The horizontal periodicities of the PMCs can remain stable for at least 24 hours.